What's in your urine?

You’ve heard of the genome, which is the set of all genes within a cell. In the same way, the ‘metabolome’ is the set of all small metabolites found within a sample. Thanks to the work of University of Alberta researchers led by Souhaila Bouatra, we’re closing in on the human urine metabolome. I know that was keeping you up at night.

Urine is surprising complex and medically relevant. Many diseases can be diagnosed by testing the levels of glucose, protein, nitrates or other molecules within urine, often with a simple dipstick test. You could hardly ask for a less invasive test. Therefore, the more comprehensive the list of all the components found in urine, together with the consequences for deviating from normal ranges, the better.

The authors were able to identify hundreds of unique metabolites within the urine samples using the following techniques:

• high-resolution nuclear magnetic resonance (NMR) spectroscopy
• gas chromatography mass spectrometry (GC-MS)
• direct flow injection tandem mass spectrometry (DFI/LC-MS/MS) 
• inductively coupled plasma mass spectrometry (ICP-MS)
• high performance liquid chromatography (HPLC) with ultraviolet (UV)
• (HPLC) with fluorescence detection (FD) techniques

Venn diagram showing the overlap of urine metabolites (compounds or cmpds) detected by NMR, GC-MS, DFI/LC-MS/MS, ICP-MS, HPLC coupled to UV detection and HPLC coupled to fluorescence detection compared to the detectable urine metabolome.

doi:10.1371/journal.pone.0073076.g005.

I think it’s safe to say that the researchers were extremely thorough in their search for urine metabolites. Even though the urine metabalome should be a subset of the blood metabalome (everything in urine was originally pulled out of the bloodstream by the kidneys), the researchers found many compounds not previously found in blood. Most likely, these metabolites are present below the threshold of detection in blood but become highly concentrated in the urine.

You can see the complete list of all the urinary compounds identified, from this study and from a literature search, at the Urine Metabalome Database. If you’re so inclined, you can also browse through chemical structures, maps of metabolic pathways, or clinical data.

The authors offer this fun fact:

The average adult generates between 1.5–2.0 liters of urine per day, which over the course of their lifetime would be enough to fill a small backyard swimming pool (5 X 8 X 1.5 m).

Bouatra S, Aziat F, Mandal R, Guo AC, Wilson MR, Knox C, Bjorndahl TC, Krishnamurthy R, Saleem F, Liu P, Dame ZT, Poelzer J, Huynh J, Yallou FS, Psychogios N, Dong E, Bogumil R, Roehring C, & Wishart DS (2013). The human urine metabolome. PloS one, 8 (9) PMID: 24023812.